STM32F413 fullduplex I2S?

Up to 5 SPI/I2Ss (up to 50 Mbit/s, SPI or I2S audio protocol), out of which 2 muxed full-duplex I2S interfaces

Similar text can be found at the beginning of datasheet. The I2S chapter, though goes even further, explicitly stating (DocID029162 Rev 3, 3.26 Inter-integrated sound (I2S)):

Five standard I 2 S interfaces (multiplexed with SPI1 to SPI5) are available. They can beoperated in master or slave mode, in simplex communication mode, and full duplex modefor I2S2 and I2S3.

Besides that I have no idea what "muxed full-duplex" might mean, I can't find a single mention of full-duplex I2S in RM0430 rev.2; contrary, the I2S subchapter is very specific in that it's half-duplex only.